1. Technical Field of the Invention
The present invention relates in general to the mobile communications field and, in particular, to a method for controlling transmission power levels in a spread spectrum or Code Division Multiple Access (CDMA) cellular communication system.
2. Description of Related Art
In wideband spread spectrum cellular communication systems (often referred to as Wideband-CDMA or WCDMA systems), power control symbols are transmitted on a regular basis in order to compensate for the effects of changing conditions that influence the channel, such as fading and shadowing. For example, a base station can transmit a power control command which signals a served mobile station to increase or decrease its transmission power level. The mobile station can then decide whether to increase or decrease its transmission power level in response to the power control command received.
As such, it is an important premise that the power control algorithms used in CDMA systems be designed to maintain the negotiated quality of the data channels for all active users. Essentially, the basic power control algorithms used in existing systems are designed to implement this capability in each connection, with two nested control loops. The outer (slower) power control loop controls a transmit signal-to-interference ratio (SIR) or signal-to-noise ratio (SNR) target value, which is used in the inner (faster) closed power control loop to maintain the actual Quality of Service (QoS) as close as possible to the negotiated QoS. The inner power control loop estimates the SIR of the forward transmission channel, compares the estimated SIR to the SIR target value from the outer control loop, and based on the results of the comparison, transmits certain power control commands on the reverse transmission channel which xe2x80x9cadvisexe2x80x9d the transmitter on the forward channel about whether to increase or decrease the transmission power level. Notably, the same power control principles are applicable for controlling both uplink and downlink transmissions.
The inner power control loops used in existing systems are designed to function based on SIR measurements that can be obtained (typically for WCDMA systems) at a rate of 1500 per second. The outer power control loops used in existing systems are designed to function based on QoS values that can be obtained at a rate of one per second. Typically, these QoS values are expressed in terms of a frame error rate (FER), bit error rate (BER), packet delay, or speech quality index (SQI). As such, these QoS values can be obtained only at the above-described rate of once per second, or once per code block (typically, 50 to 100 times per second). However, these QoS values should be accumulated over a significant period of time in order to produce statistically reliable results.
The outer power control loops used in existing systems can change a SIR target value faster for a specified amount of change (e.g., a change related to a service, such as a transmission data rate change). As such, for a properly designed power control implementation, the outer power control loop should be able to vary the SIR target value no faster than the inner power control loop can respond and change the actual SIR. This approach ensures that the actual SIR will follow the SIR target value quite closely all of the time.
Generally, during start up of a typical radio communication system (prior to establishing a link), the usual approach followed is to develop and use information about how the transmission environment and coding scheme affect the SIR requirement, in order to set the initial SIR target value. For WCDMA systems, a number of different approaches have been proposed for controlling the SIR target value after start up (once a link has been established). For example, commonly-assigned U.S. patent application Ser. No. 09/300,079 describes three such approaches, each of which assumes that it is possible to obtain the frame errors from the data channel in order to derive a SIR target adjustment value that can be added to the SIR target value. As such, these approaches primarily address the problem whereby an inner loop is unable to maintain the SIR target value calculated by the outer loop, which causes the outer loop to change the SIR target value unnecessarily.
Commonly-assigned U.S. patent application Ser. No. 09/344,121, titled xe2x80x9cPower Control Based On Combined Quality Estimates,xe2x80x9d describes another approach for controlling the SIR target value in a WCDMA system. Although it is mentioned that the system may combine frame error measurements from the data channel with bit error measurements in order to adjust the SIR target value, no specific method for combining these measurements is disclosed.
As illustrated above, some significant power control problems need to be resolved for WCDMA systems. For example, a method for combining different types of quality indicators at different rates is needed, which can be used to control SIR target values in a WCDMA system. Also, a method for determining how to use the numerous different formats required for different services is needed. For example, some services provide a Cyclic Redundancy Check (CRC) flag for every decoded data block. This flag indicates whether or not the data block was correctly decoded. Other services provide a number indicating the reliability of the decoded data blocks. As such, a large number of service formats can be used. In any event, there is no method available for an outer loop control implementation to use quality indicators for controlling SIR target values other than using CRCs. Nevertheless, as described in detail below, the present invention successfully resolves these and other related problems.
In accordance with a preferred embodiment of the present invention, a method and apparatus for controlling transmission power levels in a WCDMA cellular communication system are provided, whereby a third (middle) control loop is interposed between an inner loop and an outer loop to form a cascaded power control loop. Certain quality indicators are coupled to the outer loop and middle loop. The outer loop produces a quality target value for the middle loop. The middle loop produces the SIR target value.
An important technical advantage of the present invention is that a WCDMA power and quality control system is provided, whereby direct (negotiated) and indirect (mapped to negotiated) quality estimate information can be used together to produce a SIR target value.
Another important technical advantage of the present invention is that a WCDMA power control system is provided, whereby a SIR target value can be produced even if quality indicators for a service are missing or can be calculated only for intermittent periods.
Still another important technical advantage of the present invention is that a WCDMA power control system is provided, whereby prior knowledge of how the transmission environment and coding scheme can affect the SIR requirements can be used to produce a SIR target value.